1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus which can perform ECG gated reconstruction.
2. Description of the Related Art
An X-ray computed tomography apparatus provides information about a subject to be examined in the form of images on the basis of the intensities of X-rays transmitted through the subject, and serves as an important role in many medical activities such as disease diagnosis, medical treatment, and surgical operation planning.
In examination of the heart which moves fast, ECG gated scanning is performed. ECG gated scanning is performed to acquire ECG gated signals (trigger signals and R wave signals) and electrocardiographic signals (ECG signals) in parallel with scanning and obtain an image synchronous with a cardiac phase by using the electrocardiographic waveform signals or the like after data acquisition.
Recent cardiac examination attempts to reduce exposure by applying X-rays only in a specific phase by using electrocardiographic information.
According to Jpn. Pat. Appln. KOKAI Publication No. 2000-342577, there is no description about how an X-ray application period is determined. According to Jpn. Pat. Appln. KOKAI Publication No. 2000-51208, an X-ray application period at the time of scanning is determined by using an R-R wave interval (cardiac cycle) corresponding to one heartbeat which is acquired before scanning. This technique may be applicable to a heart phantom with a constant cardiac cycle. However, the actual cardiac cycle of a subject (patient) is unstable, and a cardiac cycle corresponding to several heartbeats is not constant even before scanning. It is therefore not practical to determine an X-ray application period by using only a cardiac cycle corresponding to one heartbeat. In cardiac examination which is performed for several to several ten seconds, all a plurality of cardiac cycles at the time of scanning are not completely equal to one cardiac cycle before the scanning. For this reason, it may happen that no X-rays are applied (no data can be obtained) in a phase which the operator wants to reconstruct, and data in an unnecessary period is acquired.
As described above, an X-ray computed tomography apparatus provides information about a subject to be examined in the form of images on the basis of the intensities of X-rays transmitted through the subject, and serves as an important role in many medical activities such as disease diagnosis, medical treatment, and surgical operation planning. In examination of a fast-moving part using an X-ray computed tomography apparatus, and in particular, cardiac examination, one of the important challenges is to improve the time resolution of images. A direct method of achieving this challenge is to shorten the time taken for one rotation of an X-ray tube, i.e., to speed up scanning. Speeding up scanning makes a so-called ECG gated scanning method effectively function, which acquires only data in a specific period in a cardiac cycle by performing scanning in synchronism with an electrocardiogram. X-ray are generated only in a specific period or the intensity of X-rays is increased in a specific period. This makes it possible to reduce an exposure dose as compared with a case wherein X-rays are continuously applied.
However, arrhythmia or the like sometimes causes a situation in which projection data necessary for image reconstruction in a desired cardiac time phase has not been acquired, or projection data has not been acquired with a high S/N ratio. In such a situation, the image quality of a reconstructed image deteriorates. In some case, re-scanning is required. Furthermore, there is no means to check the above situation before reconstruction.